An image sensor system has a pixel array with a plurality of pixels, each of the pixels comprising a photodiode, a pixel buffer and a transfer gate coupled between the photodiode and an input of the pixel buffer. A voltage supply block is configured to generate a pixel supply voltage from an input voltage based on a first reference voltage and to provide the pixel supply voltage to the pixel array. A calibration processing block is configured to determine an average pixel signal based on an average of individual pixel signals at outputs of the pixels of the pixel array and to determine a correction value based on the average pixel signal and a reference pixel signal. A correction processing block is configured to determine the first reference voltage based on a combination of a second reference voltage and the correction value.